QUESTIONÁRIO GERAL 002EE2353.doc

7/28/2019 QUESTIONRIO GERAL 002EE2353.doc

1/15

OM SATHI

ADHIPARASAKTHI COLLEGE OF ENGINEERING

3.1 SYLLABUS

EE 2353 HIGH VOLTAGE ENGINEERING 3 0 0 3

1. OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS 6

Causes of over voltages and its effect on power system Lightning, switching

surges and temporary over voltages - protection against over voltages

2. ELECTRICAL BREAKDOWN IN GASES, SOLIDS AND LIQUIDS

10

Gaseous breakdown in uniform and non-uniform fields corona discharges

Vacuum breakdown - conduction and breakdown in pure and commercial liquids

breakdown mechanisms in solid and composite dielectrics.

3. GENERATION OF HIGH VOLTAGES AND HIGH CURRENTS 10

Generation of High DC, AC, impulse voltages and currents. Tripping and control of

impulse generators.

4. MEASUREMENT OF HIGH VOLTAGES AND HIGH CURRENTS 10

Measurement of High voltages and High currents digital techniques in high

voltage measurement.

5. HIGH VOLTAGE TESTING & INSULATION COORDINATION 9

High voltage testing of electrical power apparatus power frequency, impulse

voltage and DC testing International and Indian standards Insulation

Coordination.

L = 45 Total = 45

TEXT BOOK

1. M.S. Naidu and V. Kamaraju, High Voltage Engineering, Tata McGraw Hill, 3rd

Edition, 2004.

REFERENCE BOOKS

1. E. Kuffel and W.S. Zaengl, High Voltage Engineering Fundamentals, Pergamon

press, Oxford, London, 1986.

2. E. Kuffel and M. Abdullah, High Voltage Engineering, Pergamon press, Oxford,

1970.

EE2353 - HIGH VOLTAGE ENGINEERING


2/15

OM SAKTHI

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

3.2 SHORT QUESTIONS AND ANSWERS

UNIT-1

1. What are the 2 types of over voltages?

Lightning over voltages, Switching over voltages.

2. Explain the various regions of the cloud.

The upper regions of the cloud are positively charged, whereas the lower region and

the base are predominantly negative except the local region near the base and the head

which is possible.

3. Mention the different theories of charge formation.

Simpsons theory, Reynoldss theory and Masons theory

4. What does a thunder cloud consist?

A thunder cloud consists of super cooled water droplets moving upwards and large

hailstones moving downwards.

5. Mention the requirements for the thunder clouds and charge formation of air

currents.

Moisture and specific temperature range

6. What is back flashover?

When a direct lightning stroke occurs on a tower, the tower has to carry huge impulse

currents. If the tower footing resistance is considerable, the potential of the tower rises to a

large value, steeply with respect to the line and consequently a flashover may take place

along the insulator strings .This is known as back flashover.

7. State the parameters and the characteristics of the lightning strokes.

Amplitude of the current, the rate of rise, the probability distribution of them andthe wave shapes of the lightning voltages and currents.

8. Define Isokeraunic level or thunderstorm days.

It is defined as the number of days in a year when the thunder is heard or recorded

in a particular location. Often it does not distinguish between the ground strokes and the

cloud-to-cloud strokes.

9. State the factors influence the lightning induced voltages on transmission lines.

The ground conductivity, the leader stroke current and the corona.

3rdYEAR/ 6th SEMESTER


3/15

OM SATHI


10. State the attenuation and distortion of travelling waves.

The decrease in the magnitude of the wave as it propagates along the line is called

attenuation. The elongation or change of wave shapes that occur is called distortion.

11. When over voltages are generated in EHV system?

Over voltages are generated in EHV systems when there is a sudden release of internal

energy stored either in the electrostatic form or in the electromagnetic form.

12. What are the causes for power frequency and its harmonic over voltages?

Sudden loss of loads, Disconnection of inductive loads or connection of capacitive

loads, Ferranti effect, unsymmetrical faults and saturation in transformers etc.

13. What are the uses of shunt reactors?

-used to limit the voltage rise due to Ferranti effect.

-used to reduce surges caused due to sudden energizing.

14. What is ground wire?

Ground wire is a conductor run parallel to the main conductor of the transmission

line supported on the same tower and earthed at every equally and regularly spaced towers.

It is run above the main conductor of the line.

15. What is the use of ground wire?

It shields the transmission line conductor from induced charges, from clouds as well

as from a lightning discharge.

16. What is an expulsion gap?

Expulsion gap is a device which consists of a spark gap together with an arc

quenching device which extinguishes the current arc when the gap breaks over due to

overvoltage.

17. Mention the parts of an expulsion gap.

It consists of a rod gap in air in air in series with a second gap enclosed within a

fiber tube.

18. What is a protector tube?

It is a device which consists of a rod or spark gap in air formed by the line

conductor and its high voltage terminal. It is mounted underneath the line conductor on a

tower.

19. How are the insulation level and the protective safety margin arrived?



4/15

OM SAKTHI


Selecting the risk of failure, the statistical safety factor and by firing the withstand

level of any equipment or apparatus corresponding to 90% or 95% of the withstand voltage.

20. Define Basic Impulse Level.

It is defined as the minimum insulation impulse withstands voltage of any power

equipment or apparatus. The BIL of a power system is usually chosen as 25% to 30% morethan the protective level offered by the protective devices.

21. State the main disadvantages of Zinc Oxide arrester.

It is the continuous flow of power frequency current and the consequent power

loss.

22. Mention the various insulation levels in a substation?

The bus bar insulation is the highest to ensure the continuity of supply in a

substation. The circuit breakers, isolators, instrument and relay transformers are given thenext lower limiting level. The power transformers are the costliest and sensitive device and

the insulation level for it is the lowest.

23. What are surge arresters?

They are non-linear resistors in series with spark gaps which act as fast switches.

24. What are the various types of surge arresters used for EHV and UHV systems?

Silicon carbide arresters with spark gaps, Silicon carbide arresters with current

limiting gaps and the gapless metal oxide arresters.

25. Write the equation of surge admittance and surge impedance of the transmission

line.

Y(S) = C/L ((S+ - ) (S+ + ))1/2

Z(S) = L/C ((S+-)(S+-)) 1/2

Where is the attenuation constant and is the wavelength constant.

UNIT-2

1. Mention the gases used as the insulating medium in electrical apparatus?

Most of the electrical apparatus use air as the insulating medium and in a few cases

other gases such as nitrogen, carbon dioxide, Freon and sulphurhexafluoride

2. What is breakdown voltage?

The maximum voltage applied to the insulation at the moment of breakdown is

called the breakdown voltage.

3. What are the two types of electrical discharges in gases?

Non-sustaining discharges, self-sustaining discharges.



5/15

OM SATHI


4. What is spark breakdown?

Spark breakdown is the transition of a non-sustaining discharge into a self-

sustaining discharge.

5. State the two types of theories which explain the mechanism for breakdown.

Townsend theory and streamer theory

6. What are collision processes?

Collision process is mainly gas processes which occur due to the collision

between the charged particles and gas atoms or molecules.

7. What are the two types of collision?

Elastic collision and Inelastic collision

8. What are elastic collisions?

They are collisions which when occur, no change takes place in the internal energy

of the particles but only their kinetic energy gets redistributed.

9. What are inelastic collisions?

They are those in which internal changes in energy takes place within an atom

or a molecule at the expense of the total kinetic energy of the colliding particle. The

collision often results in a change in the structure of the atom.

10. Give examples of inelastic collisions.

Ionization, attachment, excitation and recombination.

11. What is diffusion?

When particles possessing energy, which is exhibited as a random motion aredistributed unevenly throughout a space, then they tend to redistribute themselves

uniformly throughout the space. This process is known as diffusion.

12. Define collision cross section.

It is defined as the area of contact between two particles during a collision. In other

words the total area of impact.

13. Define the mean free path.

It is defined as the average distance between collisions. The mean free path is

expressed as k/p cm , where k is a constant and p is the gas pressure in microns.



6/15

OM SAKTHI


14. What is ionization?

The process of liberating an electron from a gas molecule with a simultaneous

production of a positive ion is called ionization.

15. What is Townsends first ionization coefficient?

Townsends first ionization coefficient is the average number of ionizing collisions

made by an electron per centimeter travel in the direction of the field.

16. What is Townsends secondary ionization coefficient?

The Townsends secondary ionization coefficient is defined as the net number of

secondary electrons produced per incident positive ion,photon,excited particle or meta

stable particle.

17. What is an electronegative gas?

An electronegative gas is one in which the electrons get attached to form negative

ion.

18. Define an attachment coefficient.

An attachment co-efficient is defined as the number of attaching collusions made

by one electron drifting one centimeter in the direction of the field.

19. What is meant by time lag?

The time difference between the application of a voltage sufficient to cause

breakdown and the occurrence of breakdown itself is called as time lag.

20. What is a statistical time lag of the gap?

The time which lapses between the application of the voltage sufficient to cause

breakdown and the appearance of the initiating electron are called a statistical time lag of

the gap.

21. Mention the different mechanisms for the breakdown in vacuum.

Partial exchange mechanism, Field emission mechanism and Clump theory.

22. Write the two mechanisms used in field emission theory?

Anode heating and cathode heating mechanisms.

23. What are the classifications of vacuum?

High vacuum, Very high vacuum and Ultra high vacuum

24. Why are liquid dielectrics?



7/15

OM SATHI


Liquid dielectrics are normally mixtures of hydrocarbons and are weakly polarized.

25. Mention some of the applications of liquid dielectrics.

They are used as impregnates in high voltage cables and capacitors, and for filling

up of transformers, circuit breakers .They are also used as heat transfer agents in

transformers and as arc quenching media in circuit breakers.

26. Name some examples of liquid dielectrics.

Petroleum oils, Synthetic hydrocarbons, halogenated hydrocarbons, silicone oils

and fluorinated hydrocarbons.

27. What are pure liquids? Give examples.

They are chemically pure and do not contain any other impurity even in traces of 1in 109 and are structurally simple. Examples are n-hexane-heptanes and other paraffin

hydrocarbons.

28. Mention the impurities added in liquid dielectrics.

Dust, moisture, dissolved gases and ionic impurities.

29. What are the different types of solid insulating materials?

Organic materials: paper, wood and rubber

Inorganic materials: Mica, glass and porcelainSynthetic polymers : Persplex, PVC, epoxy resins

30. State the properties of good dielectrics

Low dielectric loss, high mechanical strength, should be free from gaseous

inclusions and moisture and be resistant to thermal and chemical deterioration.

31. Name the two types of intrinsic breakdown mechanisms.

Electronic breakdown. Avalanche or streamer breakdown.

UNIT-3

1. Specify the rectifier circuits for producing high dc voltages from ac sources.

Half wave and full wave rectifiers.

2. What are the classifications of high voltages?

High dc , high ac of power frequency , high ac of high frequency and impulse

voltages.



8/15

OM SAKTHI


3. What is regulation?

The change of average voltage across the load from the no load theoretical value

expressed as a percentage of no load is called regulation.

4. What is a tesla coil?

The high frequency resonant transformer is called as a tesla coil.

5. What are the uses of high frequency high voltages?

They are required for rectifier dc power supplies. Also for testing electrical

apparatus for switching surges it is used.

6. Mention the advantages of high frequency transformers.

Saving in cost and size, pure sine wave output , uniform distribution of voltage

across the winding coils due to subdivision of coil stack into number of units.

7. How are impulse waves specified?

By defining the rise of front time, fall times to 50% peak value and the value of the

peak voltage.

8. Mention the circuits to produce impulse waves.

In the laboratory with a combination of a series R-L-C circuit under over damped

conditions or by the combination of 2 R-C circuits.

9. Name the multi test sets used for high voltage testing.

Ac testing transformers, dc units, impulse voltage units .

10. What is transient voltage?

It is an oscillatory wave or a damped oscillatory wave of frequency ranging for few

hundred hertz to few kilo hertz.

11. What are the components of a multistage impulse generator?

Dc charging unit, charging resistors, generator capacitors and spark gaps , wave

shaping resistors and capacitors , triggering system , voltage dividers and gas insulated

impulse generators.

12. Define the duration of the wave.

It is defined as the total time of the wave during which the current is at least 10% ofits peak value.

13. How are impulse currents of large value produced?



9/15

OM SATHI


A bank of capacitors connected in parallel are charged to a specified value and are

discharged through a series R-L circuit.

14. How will you generate rectangular current pulses with high magnitudes?

They are generated by discharging a pulse network.

15. Mention the advantage of trigatron gap?

It requires much smaller voltage for operation compared to the three electrode gap.

16. What are the components of a trigatron gap?

It consists of a high voltage spherical electrode of suitable size, an earthed main

electrode of spherical shape and a trigger electrode through the main electrode.

UNIT-4

1. Mention the techniques used in impulse current measurements.

Hall generators, Faraday generators and current transformers.

2. Mention the problems associated with bifilar strip design.

The shunt suffers from stray inductance associated with resistance element and its

potential leads are linked to a small part of the magnetic flux generated by the current thatis measured.

3. Mention the different ways in which the stray effect is reduced in resistance shunt?

Bifilar flat strip design, Co-axial tube or parks shunt design and Co-axial squirrel cage

design.

4. Specify the 2 types of arrangements in sphere gaps.

Vertically with lower sphere grounded and horizontally with bith spheres connectedto the source voltage or one sphere grounded.

5. State the advantages of Sphere gaps?

They are used for voltage measurements. They are suitable for all types of

waveforms from d.c to impulse voltages of short times. They are used for radio frequency

a.c voltage peak measurements up to 1 MHz.

6. State the advantages of magnetic potential transformers.

They are simple in construction and designed for any voltage. For very high voltagescascading of the transformers are possible.



10/15

OM SAKTHI


7. How is an electric field is measured?

It is measured by introducing a small fixed capacitance probe into the field area and

measuring the induced charge on it.

8. Mention the devices used to measure the d.c electric field strength.

Variable capacitor probe and a vibrating plate capacitor.

9. Give the advantages of generating voltmeters

No source loading by the meter. No direct connection to high voltage electrode .Scale

is linear and extension of range is easy.

UNIT-5

1. Define routine test.

They are intended to check the quality of an individual test piece. They are done to

ensure the reliability of an individual test object and quality and consistency of materials

used in their manufacture.

2. Define withstand test voltage.

The voltage which has to be applied to a test object under specified conditions in a

withstand test is called withstand voltage.

3. Define flashover voltage.

The voltage that causes a flashover at each of its applications under specified

conditions when applied to test objects as specified is called flashover voltage.

4. Name the tests conducted on surge diverters.

Power frequency spark over test,Hundred percent impulse spark over test, Front ofwave sparkover test and residual voltage test.

5. Give any two classification of test on cables.

Partial discharge test and impulse withstand voltage test.

6.Explain the porosity test on insulators.

The insulator is broken and immersed in a 0.5 % alcohol solution under a pressure

of 13800 KN/sq.m for 24 hours. The broken insulator is taken out and further broken. It

should not show any sign of impregnation.



11/15

OM SATHI


7. Why is the cable meant for a.c system to be tested with dc supply?

Cables are tested for power frequency ac and dc voltages. During manufacture the

entire cable is passed through a higher voltage test and the rated voltage to check the

continuity of cable.High voltage dc of 1.8 times the rated dc voltage of negative polarity for

30 minutes is applied and cable is set to have no failure.

8. What is a surge diverter?

It is a non-linear resistor in series with a spark gap kept at line terminals in the

substations.

9. Define creeping distance.

It is the shortest distance on the contour of the external surface of the insulator unit .

10. What is the importance of radio interference voltage measurements for extra high

voltagepower apparatus?

In the power apparatus it produces unwanted electrical signals in radio and high

frequency ranges.It is important to see that the noise generated should be reduced.For this

purpose this measurement is important.

11. Name the test conducted on bushings.

Power frequency test ,impulse voltage test and thermal test.

12. Mention the applications of HVDC in high voltage testing.

In cables during manufacture entire cable is passed throygh a high voltage test to

check its continuity.In insulators to check the design features and quality it is used.

13. Define partial discharge.

An electrical discharge that only partially bridges the dielectric between the

conductors.Examples are surface discharge,internal discharge

14. What is treeing?

The spreading of spark channels during tracking in the form of branches of a tree is

called treeing.

15. What is the function of surge arrester?

They are capable of discharging 10 to 20 KA of long duration surges and 100 to 250

KA of short duration surge currents.

16. State the consequences of RIV.



12/15

OM SAKTHI


When the noise meter measurements are stated the information regarding the

specification of meters used in the band pass characteristics and the open circuit the

detector characteristics has to be mentioned.

17. What do you mean by radio interference?

The power apparatus produces unwanted signals in the radio and high frequencyranges. These are called radio interference.

18. Define an isolator.

It is a disconnector or a mechanical switching device which provides in the open

position an isolating distance in accordance with special requirements.

19. Define a circuit breaker.

It is a switch which automatically interrupts the circuit when a critical current or

voltage rating is exceeded.

20. Define 50% flashover.

It is the voltage which has the 50 % flashover when applied to test object.

21. Define 100 % flash over.

The voltage that causes a flashover at each of its application under specified

conditions when applied to test objects as specified.

22. State 2 standard tests to be conducted on HV Transformers.

Induced overvoltage test and partial discharge test

23. What does the selection of BIL level for lines depend?

Atmospheric conditions, lightning activity, insulation pollution and acceptable outage

of the line.

24. Define type test.

They are intended to prove or check the design features and the quality.They are done

on samples when new designs or design changes are introduced.

PART-B [16MARKS]

UNIT-I

1. Explain the mechanism of lightning strokes including high over voltages on

transmission line(16)

2. Mention the limitations of using protector tubes for protection against over voltage?(4)?



13/15

OM SATHI


3. Draw a cross sectional views of a non-lineart resistor lightning arrestor and explain its

operation. Give a typical of L.A.(12)

4. Derive the mathematical model for lightning discharges and explain them(10)

5. Explain the process for power frequency over voltages(6)

6. Explain different methods employed for lightning protection(16)

7. List the drawbacks of expulsion type L.A.(4)

8. Explain about protection of transmission line using search diverters(6)

9. Explain various methods to control switching over voltages(10)

10. What is meant by coefficient of earthing. Explain its implication on the design of

insulation of systems (16)?

11. Explain with sketch the various theories of charge generation and discharging a

thunder cloud?

12. Explain what is meant by insulation coordination how are protective devices chosen

For optimal insulation level in a power system(16)

UNIT-II

1. What are the factors that influence conduction in pure liquid dielectric and in

commercial liquids dielectric(8)

2. Discuss the various mechanism of vaccum breakdown(8)

3. Describe the mechanism of short terms (i) Break down of composte insulations(8)

4. Discuss the current growth in a gas subjected to uniform and non-uniform electric

field(8)

5. Discuss Meeks theory of breakdown in gases under (i) non-uniform fields (8)

(ii) Discuss the phenomenon of thermal break down in solid dielectrics(8)

6. (i)Define intrinsic strength of a solid dielectic material. Explain why intrinsic strength is

not fully realized in practice(8)

(ii)State why the very high intrinsic strength of solid dielectrics is not fully realized in

practice. Discuss in detail any one mechanism of breakdown in solid dielectrics

7. Explain various theories which explains break down in commercial liquid dielectrics

8. (i)What is electrical avalanche? How do avalanche give rise to an electrical breakdown

in case of town sends type of discharge (12)

(ii)Derive from the basic the condition for break down in town sends discharge(6)9. State the criteria for sparking potential and hence obtain the relation between sparking

potential and (pd) values (paschens law). Discuss on the nature of variations of sparking

potential with (pd) values(16)

10. Explain the different mechanism by which break down occurs in solid dielectrics in

practice(16)

11. A Coaxial capacitor is to be designed with an effective length of 20 cm, the capacitor is

expected to have a capacitance of 1000 PF and to operated 15KV, 500 HZ. Find the

dimension of electrodes assume the dielectric material is having relative permeability of

2.3.12. Define town sends first and second ionization coefficients. How is the condition for

BD obtained in town sends discharge(16)



14/15

OM SAKTHI


13. Deduce Townsends BD criteria. Also define the Townsends primary and secondary

ionization coefficients(16)

14. Explain the characteristics of liquid dielectrics(16)

15. What are preferred properties gaseous dielectrics for high voltage applications? Draw

the characteristics of DC BD strength of typical solid, liquid, gas, and vaccum insulators in

uniform fields(16)?

16. Explain clearly BD in non-uniform fields and corona discharges?(16)

UNIT-III

1. What are the different forms of high voltage classified? Explain any one method of

voltage multiplier circuit (16)

2. Draw and explain the circuits for producing impulse waves(16)

3. Discuss with diagrams the operation of van de Graff generator(16)4. Describe the cascade transformer connection to generate high alternating voltages?

Discuss its limitations and features(16)

5. With a neat circuit diagram explain the working principle of a working cock craft

Walton voltage multiplier circuit.

6. Give the Mak-circuit multistage impulse generator. How the basic arrangements are

modified to accommodate the wave time controlled resistance?

7. What is the tesla coil? How are dammed high freq. oscillations obtained form a tesla

coil.(16)

8(i) Describe the circuit arrangement for producing lightning current waveforms in lab(10)(ii) Explain with neat sketch the principles of operation of resonant transformer for

generating HVAC(6)

9(i).Explain tripping and impulse generators

(ii)An impulse generator has 12 capacitors of 0.12 MF and 200kv rating. The wave front

and wave tail resistances are 1.25 Kohm and 4 kohm respectively. If the load

capacitance including that of the test object is 10,000 pF., find the wave front and

wave tail times and the peak voltages of impulse wave produced (6).

(iii)Explain the need for generating high DC voltages (4).

10.(i) what is the importance of control tripping of impulse generators? How is it done

using trigatron gap )

(ii) Explain the methods of generating switching surges in laboratories?

11. Calculate the peak current and wave shape of the output current of the following

generator, the total capacitance of the generator is 53 mf. The charging voltage is

240kv. The circuit inductance is 1.54 mh. & the dynamic resistance of the test object is 0.05

ohm.

UNIT-IV

1. Explain how a sphere gap can be used to measure the peak value of voltages.

2. What are the parameters and factors that influence such voltage measurements?3. Compare the merits and demerits of measuring



15/15

OM SATHI


4. (i)Verify high voltage using sphere gaps and potential dividers (8)

(ii)describe the generating voltmeter method for measuring high DC voltages and state its

merits and demerits (8)

5. Define 50% disruptive discharge as applied to impulse voltages. Mention the methods

adopted to obtain 50% disruptive discharge voltages and discuss any one method in detail.

(16)

6. Explain the working principle and operation of electrostatic voltmeter?(16)

7. What are capacitance voltage dividers? Explain

(i)various capacitance voltages dividers used to measure impulse voltages up to 2MV(10)

(ii)Explain the methods of measuring high DC currents (6)

8. What is CVT? Explain with phasor diagram how a tuned capacitance voltage

transformer can be used for voltage measurement in power system?9. Discuss various techniques for the measurement of impulse voltage

10. Tabulate High voltage & High current measurement techniques for different types

of voltages and currents (16)

UNIT-V

1. (a)How are the protective devices chosen for the optimal insulation level is a power

system (10)

(b)Explain the following terms:-

(i)Withstand voltage

(ii)Flashing voltage

(iii)50% flashing voltage

2. Explain the impulse testing procedure for insulators (16)

3. Explain the need for high voltage testing of (i)Electrical apparatus (4)

(ii)Mention the different types of nature of test conductors. (4)

(iii)Discuss the arrangement with detailed procedure for conducting wrt withstand test and

state specification for water used in such tests. (8)

4. (i)Explain the synthetic testing of circuit breakers. (8)

5. (ii)Explain with schematic diagram any one method of measuring RIV of transmissionline (8)

6. What are the tests done on cables? How samples (i) are prepared? Explain any two tests

(ii)Explain long duration impulse current test and operating duty cycle test on surge

diverters (8)

7. Give detailed procedure for impulse voltage withstand test on a 500KVA, 11KV/415V

delta/star distribution transformer. Explain the need for recording supplementary

oscillograms during impulse testing of transformers. (16)

8. What are the different power frequency tests on bushings? Mention the procedure for

testing? Discuss the various tests carried out in a CB at HV labs.


QUESTIONÁRIO GERAL 002EE2353.doc

Documents

Transcript of QUESTIONÁRIO GERAL 002EE2353.doc